The invention relates generally to the field of variable resistance devices and their use in pressure transducer assemblies. The invention more particularly relates to an improved variable resistance device which produces a high resolution, wide range, varying resistance in response to a limited linear movement and the use of this device in pressure transducers.
The most common variable resistance device used in the prior art is a rotatably adjustable potentiometer. In such devices a variable resistance is produced in response to the rotating of a wiper arm which electrically contacts a resistive element that is located between two end electrical contacts. Such devices are economical and perform satisfactorily when a rotary motion is available for rotating the wiper arm.
However, in many applications, such as in pressure transducers having a displaceable diaphragm, a short stroke linear motion is the only movement initially produced. The conversion of this short linear motion to a rotary motion and the subsequent use of rotary motion to drive a normal potentiometer would typically require many additional components and thereby increase the cost of a pressure transducer assembly and decrease its reliability. The cost would be increased since additional components would be required for the assembly. The use of these additional components would also increase the total size of the assembly. The reliability of the assembly would be decreased since the use of additional components increases the liklihood of a probable failure or malfunction of one of the components of the assembly.
Some prior art pressure transducer assemblies comprise a wound resistive element and a wiper arm. The wiper arm has a single contact area portion which is linearly moved in a direction parallel to the central axis of the resistor winding in response to the linear movement produced by a pressure sensing diaphragm. Typically, a fixed portion of the wiper arm is always in contact with a portion of the wound resistive element. As this portion of the wiper arm slides along the resistive element, less of the resistive element will exist between the wiper arm and an end contact portion of the wound resistive element. This results in the creation of a variable resistance. Typically, the pitch of the wound resistive element is equal to its width in the direction parallel to the axis about which the resistor is wound. Thus the adjacent turns of the resistor electrically contact each other and the resistor is effectively a solid cylinder of resistive material. These prior art assemblies can not produce both a high resolution and wide range variable resistance. If the pitch of the resistor is greater than its width, severe resolution problems occur, since the resistance can then be varied only in increments of a whole number of windings.
The distance that the wiper arm travels is directly related to the linear diaphragm motion which is typically half an inch. Therefore without any amplification or conversion of the diaphragm linear motion, only a short linear movement is available for the wiper arm. Thus the prior art systems have had to compromise their resistive resolution with their requirements for implementing a large resistive change over the range of typical pressures which are to be monitored by these transducers. These range and resolution problems could be solved by the conversion of the linear diaphragm motion to a rotary motion. However, as was mentioned previously, the conversion to rotary motion would increase the cost of the assembly, increase the size of the assembly, and decrease the reliability of the assembly.
One prior art pressure transducer system uses the linear diaphragm motion to linearly move a multiturn resistive element, having a pitch equal to its width, with respect to a wiper arm contacting surface that effectively forms a contact line which is skewed with respect to the central axis upon which the resistive element is wound. This skewed orientation of the wiper contact surface and the resistive element axis results in providing a short stroke pressure transducer which does effectively amplify the diaphragm motion and produce a larger range of resistive values with a higher degree of resolution. However, the resolution and resistive range of this particular system is still limited because of limitations on the size of the transducer assembly. Since the resistive element is still effectively linearly disposed and the effective contact point has a linear travel, the resistive resolution and range in this prior system is a function of the largest linear dimension of the pressure transducer. Thus the aforementioned prior art system, while representing an improvement over previous prior art systems, still must strike a balance between the range and resolution of the resistance values produced by the transducer and a linear dimension of the transducer. Typically, this improved transducer uses over 100 turns of helically wound resistive material having contacting adjacent windings and wound about a central axis.
The basic problem in all the prior art transducers is that only a short stroke linear motion is available for actuation of any variable resistance device. Attempts to convert this linear motion into rotary motion are excessively costly and decrease the reliability of the transducer assembly. Attempts to amplify the available linear motion, such as skewing the central axis of the resistance element with respect to the contact area of the wiper arm, have generally resulted in increasing the complexity, size and cost of the pressure transducer assembly while still only providing a compromise in the resistance range and resolution produced by the pressure transducer assembly.